FIR: add getClassifierNames for FirScope

compiler/fir/tree/src/org/jetbrains/kotlin/fir/scopes/FirCompositeScope.kt

@@ -48,4 +48,8 @@ class FirCompositeScope(private val scopes: Iterable<FirScope>) : FirScope() {
     override fun getCallableNames(): Set<Name> {
         return scopes.flatMapTo(mutableSetOf()) { it.getCallableNames() }
     }
+
+    override fun getClassifierNames(): Set<Name> {
+        return scopes.flatMapTo(hashSetOf()) { it.getClassifierNames() }
+    }
 }

compiler/fir/tree/src/org/jetbrains/kotlin/fir/scopes/FirScope.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -35,6 +35,8 @@ abstract class FirScope {
     open fun mayContainName(name: Name) = true
 
     open fun getCallableNames(): Set<Name> = emptySet()
+
+    open fun getClassifierNames(): Set<Name> = emptySet()
 }
 
 fun FirTypeScope.processOverriddenFunctionsAndSelf(